Medical equipment for radiation therapy treats tumorous tissue with high energy radiation. The dose and the placement of the dose must be accurately controlled to insure both that the tumor receives sufficient radiation to be destroyed, and that damage to the surrounding and adjacent non-tumorous tissue is minimized.
Internal-source radiation therapy places capsules of radioactive material inside the patient in proximity to the tumorous tissue. Dose and placement are accurately controlled by the physical positioning of the isotope. However, internal-source radiation therapy has the disadvantages of any surgically invasive procedure, including discomfort to the patient and risk of infection.
External-source radiation therapy uses a radiation source that is external to the patient, typically either a radioisotope, such as .sup.60 Co, or a high energy x-ray source, such as a linear accelerator. The external source produces a collimated beam directed into the patient to the tumor site. External-source radiation therapy avoids some of the problems of internal-source radiation therapy, but it undesirably and necessarily irradiates a significant volume of non-tumorous or healthy tissue in the path of the radiation beam along with the tumorous tissue.
The adverse effect of irradiating of healthy tissue may be reduced, while maintaining a given dose of radiation in the tumorous tissue, by projecting the external radiation beam into the patient at a variety of "gantry" angles with the beams converging on the tumor site. The particular volume elements of healthy tissue, along the path of the radiation beam, change, reducing the total dose to each such element of healthy tissue during the entire treatment.
The irradiation of healthy tissue also may be reduced by tightly collimating the radiation beam to the general cross section of the tumor taken perpendicular to the axis of the radiation beam. Numerous systems exist for producing such a circumferential collimation, some of which use multiple sliding shutters which, piecewise, may generate a radio-opaque mask of arbitrary outline.
As part of collimating the beam to the outline of the tumor, the offset angle of the radiation beam, with respect to a radius line between the radiation source and the center of rotation of the radiation source, may be adjusted to allow the treated area to be other than at the center of rotation. Simultaneously changing the offset angle and the width of the radiation beam as a function of gantry angle allows tumorous tissue having an irregular cross-section within a plane parallel to the radiation beam to be accurately targeted. The width and offset angle of the radiation beam may be controlled by the use of a multiple-leaf collimator.
Adjustment of the offset angle, center, and size of the radiation beam at various gantry angles allows considerable latitude in controlling the dose. Nevertheless, these approaches still impart a considerable amount of undesired dose to healthy tissue, especially where the tumor is concave or highly irregular.
U.S. Pat. No. 5,317,616, which issued on May 31, 1994, describes a compensator that dynamically controls the effective intensity of rays within a radiation beam to produce a fluence profile of arbitrary shape. This ability to vary the intensity of individual rays within the beam, as opposed to simply turning them on or off, allows an advanced technique of therapy planning to be employed in which fluence profiles can be varied at each gantry angle to accurately control the dose to irregularly shaped tumors within the body. An efficient iterative approach allows precise calculation of the needed fluence profiles. Preferably, this compensator is used with a radiation source to direct radiation toward one slice at a time.
While this compensator can provide highly resolved control of the intensity of radiation received throughout a volume of a patient, as the degree of control increases, the burden on a machine operator to specify and verify dose can become increasingly burdensome. For example, specifying intensity of individual rays of a radiation fan beam at each of hundreds of different beam orientations with respect to a tumorous volume is a time Consuming and labor intensive process. The inconvenience of this complex planning protocol is exacerbated where the protocol must be followed separately for each of a plurality of patient slices. Where ray intensities have been identified for each gantry orientation and each patient slice and a test simulation indicates that the combined irradiation dose of all rays is erroneous, correction of the radiation plan would be extremely burdensome. Moreover, where a dose error has been made during a treatment session either due to poor planning or unexpected dose absorption or penetration, correction would also be difficult.